The invention relates to hydroprocessing of hydrocarbon-containing oils, particularly a combination process involving hydrotreating and hydrocracking.
During catalytic hydroprocessing, particulate catalysts are utilized to promote reactions such as cracking, desulfurization and denitrogenation. This is accomplished by contacting the particulate catalysts with a feedstock, such as a gas oil, under conditions of elevated temperature and pressure and in the presence of hydrogen so that, in the case of hydrotreating, substantial concentrations of sulfur components are converted to hydrogen sulfide and nitrogen components to ammonia, and, in the case of hydrocracking, usually under more severe conditions than hydrotreating, substantial concentrations of high molecular weight hydrocarbons are reduced to lighter, lower molecular weight hydrocarbons of lower boiling point. An advantage of hydrocracking lies in the relatively high conversion (i.e., 60 percent or more) of a sulfur-containing and/or nitrogen-containing gas oil feed, boiling for example, mostly above about 700.degree. F. (i.e., 700.degree. F.+ feed), to a relatively sulfur- and nitrogen-free commercial product of boiling point below 700.degree. F. (i.e., 700.degree. F.- product), such as naphthas and middle distillates, and more particularly, gasoline, jet fuel, diesel fuel, and mixtures thereof.
Recently, attention has been directed to producing 700.degree. F.- product at relatively low pressure conditions--a hydroprocess commonly known as "mild hydrocracking." Since the cost of constructing a hydrocracking unit operating at high pressures is quite significant and poses a major economic obstacle to its use, interest has developed in converting existing hydroprocessing units, such as hydrotreating or hydrodesulfurization units, into hydrocracking units. It is realized, of course, that hydrotreating units and the like are not normally designed for optimum hydrocracking conditions, and specifically, not for the high pressures usually employed in commercial hydrocracking, i.e., above 1,500 p.s.i.g. Nevertheless, there is still an advantage if even some hydrocracking (such as up to 35 percent) can be achieved under the low pressure constraints of typical hydrotreating or hydrodesulfurization units, and a challenge to the art is to discover mild hydrocracking processes yielding sufficient amounts and quality of the middle distillate and naphtha products to be commercially useful under such mild hydrocracking conditions. Typically, hydrocarbon products from "mild hydrocrackers" result in poor quality middle distillates, as for instance, jet fuel smoke points less than 15 mm and diesel fuel cetane indexes less than 40. Furthermore, such hydroprocesses ordinarily produce mostly feedstocks for fluid catalytic cracking (FCC) units.
In contrast to the "mild hydrocracker" units, the high conversion, high pressure hydrocracking units produce good quality middle distillates and naphthas; however, the amount of accompanying low nitrogen/sulfur-containing feedstock produced for an FCC unit by high conversion hydrocracking is decreased. The produced FCC feedstock is often the result of needless overtreating or overcracking and thus, the FCC feedstock typically contains excessive hydrogen.
FCC units are used in the petroleum industry to convert high boiling hydrocarbon feedstocks to more valuable hydrocarbon products, such as gasoline, having a lower average molecular weight and a lower average boiling point than the feedstocks from which they were derived. The conversion is normally accomplished by contacting the hydrocarbon feedstock with a moving bed of cracking catalyst particles at temperatures ranging between about 800.degree. F. and about 1,100.degree. F. The most typical hydrocarbon feedstocks treated in FCC units comprise a heavy gas oil usually containing about 1 to 3 weight percent of sulfur impurities (principally organosulfur compounds) or a hydrotreated heavy oil usually containing less than about 0.3 weight percent of sulfur, but on occasion such feedstocks as light gas oils or atmospheric gas oils, naphthas, reduced crudes and even whole crudes are subjected to catalytic cracking to yield low boiling, low sulfur-containing hydrocarbon products.
It is well known that FCC feedstocks which contain high levels of nitrogen have a deleterious effect on cracking catalysts. The nitrogen is typically present in the form of basic or neutral organic (organonitrogen) compounds, primarily aromatic compounds containing nitrogen heteroatoms such as pyridines, quinolines, and indoles, which are strongly sorbed on the acidic sites of the cracking catalyst. The nitrogen compounds react or otherwise interact with the acidic sites so as to decrease the activity of the catalyst. This deactivation results in decreased conversions and desired product proportions. Levels of nitrogen in the feedstock as small as 0.01 weight percent, calculated as the element, can result in some decrease in activity of the catalyst; however, significant deactivation is not normally encountered unless the concentration of nitrogen in the feedstock increases to above 0.05, and usually from about 0.08 to about 0.15 weight percent (or above).
In order to avoid substantial deactivation of cracking catalysts in FCC units by nitrogen or sulfur compounds contained in the feedstock, it has been standard practice to treat such feedstocks to reduce the concentration of nitrogen and sulfur compounds prior to subjecting the feedstocks to catalytic cracking. This practice also lowers the SO.sub.x (and NO.sub.x) emissions to the atmosphere during combustion of the FCC product gasoline or during regeneration of the cracking catalyst. Of the techniques employed in the past for removing the nitrogen or sulfur compounds from the feedstocks, hydroprocessing is the one most frequently used. However, in order to remove substantial quantities of residual nitrogen and sulfur, relatively high hydrogen pressures are typically required. Installation of equipment to carry out such a high pressure process requires a substantial capital investment, and the accompanying high hydrogenation pressures result in FCC feedstocks containing excessive hydrogen. The excessive hydrogen carried into the FCC unit causes overcracking of the gasoline-related molecules, and consequently, reduced amounts of gasoline are produced as lighter products, such as LPG and lighter, are formed.
A continuing aim in the petroleum refining art is to discover processing schemes which provide improved hydrocarbon product distribution. Modest or slight variations in hydroprocessing schemes can have significantly improved effects on both the quality and relative proportion of hydrocarbon products.
The petroleum refiner must balance economic considerations, such as capital expenditures (for refining equipment) and operational utility costs (for operating reactor vessels and pressurizing systems), with the quality of the desired hydrocarbon products. Accordingly, a major challenge to the art is to develop a hydroprocessing scheme utilizing hydrocracking for producing quality middle distillates and naphthas and still producing a low sulfur/low nitrogen FCC feed containing minimal hydrogen.